# Basal body temperature, explained

> The tiny temperature shift that confirms ovulation.

Category: Body literacy · 5 min read · Written by the Splash team · Sources checked October 2026
URL: https://splash.care/learn/body-literacy/bbt-explained

## Quick answer

- BBT rises slightly (about 0.3 to 0.6°C) after ovulation because of progesterone.
- Measure on waking, before moving, at the same time each day.
- It confirms ovulation afterwards, but does not predict it.
- BBT alone is not contraception.

Your [Basal body temperature](https://splash.care/learn/glossary/bbt) is your body temperature at full rest. After [ovulation](https://splash.care/learn/glossary/ovulation), [progesterone](https://splash.care/learn/glossary/progesterone) gives it a small nudge up, and it stays higher until your [next period](https://splash.care/tools/period-calculator).

**0.3 to 0.6°C** is the slight rise in resting temperature around ovulation (ACOG describes 0.5 to 1°F). A wearable study measured 0.28 to 0.56°C.

## How to take it

1. Use a thermometer that reads to two decimal places (a BBT thermometer).
2. Take it every morning as soon as you wake, before you sit up, get out of bed, eat or drink.
3. Try to measure at about the same time each day.
4. [Log it](https://splash.care/learn/cycle-basics/tracking-101) in Splash and look at the pattern over the whole cycle.

## Reading your chart

You are looking for a shift: a run of lower temperatures, then a sustained rise that lasts until your period. The rise shows [ovulation](https://splash.care/learn/phases/ovulation) has **already happened**, so BBT is great for confirming ovulation after the fact, but it cannot predict it in advance. ACOG says the most fertile days are the 2 to 3 days before the rise.

## Things that throw it off

- Illness or fever. ACOG warns BBT may not be reliable if you are unwell.
- Broken [sleep](https://splash.care/learn/movement-sleep/sleep-cycle) or a lie-in.
- [Alcohol](https://splash.care/learn/nutrition/caffeine-alcohol) the night before. In the large app study, cycles flagged for fever or high alcohol intake were handled separately.
- Travel and time-zone changes.
- Hormonal contraception that stops ovulation will also flatten the pattern.

## What about wearables?

Some wrist, finger or ring devices measure skin temperature overnight. In a study of 237 women trying to conceive, a wrist bracelet combining skin temperature, pulse and other signals could identify the [fertile window](https://splash.care/learn/glossary/fertile-window) with reasonable accuracy. But that is one study of one device, and skin temperature is not quite the same as a mouth temperature, so treat this as promising rather than proven.

> **Splash says:** keep your thermometer next to your bed so you can measure before you move a fin.

> BBT alone is not a reliable way to avoid pregnancy. The NHS says fertility awareness methods are 91 to 99% effective only when used correctly all the time, taught by a trained practitioner and combining several signs. If you do not follow the instructions exactly, it is about 76% effective, meaning around 24 in 100 will get pregnant in a year. ACOG gives a similar picture: fewer than 1 to 5 in 100 with perfect use, and 12 to 24 in 100 with typical use.

**Quick quiz:** What does a sustained BBT rise tell you?

1. Ovulation is about to happen
2. Ovulation has probably already happened
3. You are pregnant

Answer: Ovulation has probably already happened. Progesterone after ovulation causes the rise, so it confirms ovulation rather than predicts it.

## Sources

1. [ACOG: Fertility awareness-based methods of family planning](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning)
2. [NHS: Natural family planning (fertility awareness)](https://www.nhs.uk/contraception/methods-of-contraception/natural-family-planning/)
3. [Goodale et al. (2019) Wearable sensors reveal menses-driven changes in physiology and enable prediction of the fertile window, J Med Internet Res](https://pmc.ncbi.nlm.nih.gov/articles/PMC6495289/)
4. [Bull et al. (2019) Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, npj Digital Medicine](https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/)
5. [Cleveland Clinic: Ovulation](https://my.clevelandclinic.org/health/articles/23439-ovulation)

This is general information, not medical advice. Splash can’t diagnose. If you’re worried, talk to a GP, pharmacist or sexual health clinic. In an emergency call 999 (UK) or 911 (US).